In this volume, originally published in 1978, the authors survey the historical and contemporary research literature pertaining to two-dimensional visual-geometric illusions. They bring together much of the known data, summarising and evaluating theories that have been offered to explain these phenomena. Coren and Girgus provide a new conceptual framework that suggest that visual illusions are not unitary phenomena. Within this framework, illusions do not represent a breakdown in normal perceptual processing. Rather, it is proposed that each illusion is produced by a number of mechanisms operating at different levels in the visual information processing system. The book contains an extensive collection of illusion figures. It will be essential reading for all of those concerned with vision and visual perception, since it integrates the study of illusions into the main body of psychological and perceptual theories at the time.